1. Field
Aspects of embodiments of the present invention relate to a secondary battery.
2. Description of the Related Art
Generally, a secondary battery is chargeable and dischargeable and is widely used as a power source for various portable small electronic devices, such as cellular phones, personal digital assistants (PDAs), notebook computers, and the like. In particular, lithium (Li) secondary batteries having an operation voltage of 3.6V, which is about three times the operation voltage of comparable Ni—Cd or Ni-MH batteries, and exhibiting a relatively high energy density per unit weight, have been increasingly widely used.
In a typical Li secondary battery, a lithium-based oxide may be used as a positive electrode active material, and a carbon-based material may be used as a negative electrode active material. The Li secondary battery may be manufactured in various shapes, e.g., cylindrical can types, rectangular or prismatic can types, pouch types, etc.
In addition, Li secondary batteries may be classified as Li ion batteries using liquid electrolyte and polymer electrolyte batteries using polymer solid electrolyte, depending on the electrolyte used therein. Liquid polymer batteries may be classified as pure solid-type Li polymer batteries without electrolyte and gel-type polymer batteries with electrolyte, depending on the solid polymeric electrolyte used therein.
The Li polymer battery using polymeric solid electrolyte includes a pouch generally formed of a metal thin film as a receptacle of an electrode assembly, and peripheral portions of the pouch are sealed in a state in which the electrode assembly is placed into the pouch. Ends of positive and negative electrode tabs electrically connected to positive and negative electrodes of the electrode assembly are drawn out from the pouch to then be electrically connected to a protection circuit board.
In the aforementioned conventional pouch-type secondary battery, edge portions of three sides of the pouch are sealed. Specifically, the sealing of a terminal portion, that is, a top portion, of the pouch is performed linearly in an oblong shape. Here, the sealing may have both advantages and disadvantages depending on whether the sealing is performed closer to a cell space of the pouch accommodating the electrode assembly or to the drawn electrode tabs.
In the former case, an inner layer lifting phenomenon, in which an inner layer of the pouch is melted and lifted as a result of the sealing, is not exhibited outside the outer line of the pouch, which is advantageous in attaining a desired total height margin of the pouch. In this case, however, there is a risk of a separator being partially lifted and fused by the electrode tabs. In addition, several problems, including a reduction in electrolyte impregnating capability, degassing, or the like, may be undesirably caused.
By contrast, in the latter case, occurrence of fusion of the separator is avoidable. However, the inner layer lifting phenomenon may be exhibited, which is disadvantageous in attaining a desired total height margin of the pouch.